Rocky Mountain National Park entrance is 2 miles outside of Grand Lake. AMAZING mountain views,wildlife, hiking, etc.
Grand Lake, CO is a cute little mountain town with shopping and eateries a plenty. The lake is accessible from town and has rentals for kayaking and other activities at the little beach. If you're lucky, you might even get a sighting of the town Moose
Winter Park Resort is about 30 miles from Grand Lake. You will drive through Winter Park on the way to Grand Lake. They often have fun summer activities / festivals.
About 4 miles from Grand Lake is a larger lake, Lake Granby. This is a great lake to rent a boat (pontoon or regular boat) for a few hours. You can just cruise the lake and view wildlife (lots of osprey and eagles) and fish at this lake. If you are interested in renting a boat for a few hours, I recommend Beacon Landing. There are other good rental places at the lake as well if Beacon Landing is booked.
About 30 minute drive from Grand Lake are the Hot Sulphur Springs. Emphasis on "Sulphur"... but a nice relaxing experience!